Code review cleanup: cache placement/adjacency, drop dead code

- Cache choosePlacement and adjacentTile results per (id,dir); invalidate
  on addTile. Avoids O(n^3) placement scoring on every pointermove.
- setCurrent now re-renders only affected navs (prev + new) instead of
  refreshAllNav; remove refreshAllNav entirely.
- Add pointercancel handler so interrupted touch/trackpad gestures don't
  leave a stuck drag.
- Remove dead code: placeRect, photoConnections + /api/photo route and
  THUMBS const on server, dangling info var in pointerup.
- Trim .gitignore, remove .DS_Store, bump cache-bust to ?v=20.

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